Yarns of low tensile factor adapted particularly for use as fleece yarns have not been prepared heretofore by a straightforward spinning process from ordinary polymer in a manner consistent with commercial production techniques. Rather, it had been necessary to deliberately degrade the intrinsic viscosity of the polymer (see U.S. Pat. No. 3,396,446 issued Aug. 13, 1968) or to disrupt the crystalline structure by the addition of modifying monomer ingredients such as pentaerythritol (see Canadian Pat. No. 901,716 issued May 30, 1972) to achieve yarn structures of such tensile properties: typically tenacity of about 3.0 gpd and an elongation of about 30% (TE1/2=16).
It is of course known that by utilization of various spinning techniques in conventional systems, a `trade-off` as between tensile strength and elongation may be achieved at relatively constant tensile factor. However, even in these systems it is difficult to secure balanced tensile characteristics in the low tensile factor range e.g. 14 to 22, with tenacities of 2.5 to 4.0 gpd, preferably about 3.0 gpd and elongations of 25 to 40%, preferably about 30.
In the course of development, it has been determined that intermediate and high tensile fibers having properties like those prepared in a conventional lagged drawing operation can be produced directly from the spinning step by utilizing high speed takeup techniques. An improvement in such processing is effected by introducing a stress annealing step post-quench. In this approach invented by Messrs. Davis, Jaffe & Besso, the yarn is passed under tension through a heat treatment zone prior to initial windup in a high speed spin-draw sequence. The fibers so produced exhibit significant tensile development at low shrinkage and a novel morphological structure. A further description is contained in copending Appln. Ser. Nos. 400,863-4 of Messrs. Davis et al filed Sept. 26, 1973, assigned to the parent corporation of the present assignee.